As a type of hematoma, a bruise is always caused by internal bleeding into the interstitial tissues which does not break through the skin, usually initiated by blunt trauma, which causes damage through physical compression and deceleration forces. Trauma sufficient to cause bruising can occur from a wide variety of situations including accidents, falls, and surgeries. Disease states such as insufficient or malfunctioning platelets, other coagulation deficiencies, or vascular disorders, such as venous blockage associated with severe allergies[5] can lead to the formation of purpura (of which petechia and ecchymosis are smaller and larger classes of purpura) which is not to be confused with trauma-related bruising/contusion.[6] If the trauma is sufficient to break the skin and allow blood to escape the interstitial tissues, the injury is not a bruise but instead a different variety of hemorrhage called bleeding. However, such injuries may be accompanied by bruising elsewhere.[7]

Bruises often induce pain, but small bruises are not normally dangerous alone. Sometimes bruises can be serious, leading to other more life-threatening forms of hematoma, such as when associated with serious injuries, including fractures and more severe internal bleeding. The likelihood and severity of bruising depends on many factors, including type and healthiness of affected tissues. Minor bruises may be easily recognized in people with light skin color by characteristic blue or purple appearance (idiomatically described as "black and blue") in the days following the injury.

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The presence of bruises may be seen in patients with platelet or coagulation disorders. Unexplained bruising may be a warning sign of child abuse, domestic abuse, or serious medical problems such as leukemia or meningoccocal infection. Unexplained bruising can also indicate internal bleeding or certain types of cancer. Long-term glucocorticoid therapy can cause easy bruising. Bruising present around the navel (belly button) with severe abdominal pain suggests acute pancreatitis. Connective tissue disorders such as Ehlers-Danlos syndrome may cause relatively easy or spontaneous bruising depending on the severity.

During an autopsy, bruises accompanying abrasions indicate the abrasions occurred while the individual was alive, as opposed to damage incurred after death.

Bruise shapes may correspond directly to the instrument of injury or be modified by additional factors. Bruises often become more prominent as time lapses, resulting in additional size and swelling. Bruising present in a different location than the site of impact is called ectopic bruising and occurs when the tissue at the site of injury is loose, allowing blood to travel under the skin to another location due to gravity or other forces, such as in a black eye.

Genetics: Despite having completely normal coagulation factors, natural redheads have been shown to bruise more, although this may just be due to greater visibility on commonly associated lighter complexion.[8]

Bruises can be scored on a scale from 0-5 to categorize the severity and danger of the injury.[citation needed]

Bruise harm score

Harm score

Severity level

Notes

0

Light bruise

No damage

1

Mild bruise

Little damage

2

Moderate bruise

Some damage

3

Serious bruise

Dangerous

4

Extremely serious bruise

Dangerous

5

Critical bruise

Risk of death

The harm score is determined by the extent and severity of the fractures to the organs and tissues causing the bruising, in turn depending on multiple factors. For example, a contracted muscle will bruise more severely, as will tissues crushed against underlying bone. Capillaries vary in strength, stiffness and toughness, which can also vary by age and medical conditions.

An alternate bruise severity ranking system called the Chien Intensity Scale is slowly growing in popularity in some research circles. Although not widely used, the Chien Intensity Scale is used by institutes including the Ryan Mackey Memorial Research Institute and the Sydney Medical Center.

Severe bruising (harm score 2-3) may be dangerous or cause serious complications. Further bleeding and excess fluid may accumulate causing a hard, fluctuating lump or swelling hematoma. This has the potential to cause compartment syndrome as the swelling cuts off blood flow to the tissues. The trauma that induced the bruise may also have caused other severe and potentially fatal harm to internal organs. For example, impacts to the head can cause traumatic brain injury: bleeding, bruising and massive swelling of the brain with the potential to cause concussion, coma and death. Treatment for brain bruising may involve emergency surgery to relieve the pressure on the brain.

Damage that causes bruising can also cause bones to be broken, tendons or muscles to be strained, ligaments to be sprained, or other tissue to be damaged. The symptoms and signs of these injuries may initially appear to be those of simple bruising. Abdominal bruising or severe injuries that cause difficulty in moving a limb or the feeling of liquid under the skin may indicate life-threatening injury and require the attention of a physician.

Increased distress to tissue causes capillaries to break under the skin, allowing blood to escape and build up. As time progresses, blood seeps into the surrounding tissues, causing the bruise to darken and spread. Nerve endings within the affected tissue detect the increased pressure, which, depending on severity and location, may be perceived as pain or pressure or be asymptomatic. The damaged capillary endothelium releases endothelin, a hormone that causes narrowing of the blood vessel to minimize bleeding. As the endothelium is destroyed, the underlying von Willebrand factor is exposed and initiates coagulation, which creates a temporary clot to plug the wound and eventually leads to restoration of normal tissue.

During this time, larger bruises may change color due to the breakdown of hemoglobin from within escaped red blood cells in the extracellular space. The striking colors of a bruise are caused by the Phagocytosis and sequential degradation of hemoglobin to biliverdin to bilirubin to hemosiderin, with hemoglobin itself producing a red-blue color, biliverdin producing a green color, bilirubin producing a yellow color, and hemosiderin producing a golden-brown color.[7] As these products are cleared from the area, the bruise disappears. Often the underlying tissue damage has been repaired long before this process is complete.

Treatment for light bruises is minimal and may include RICE (rest, ice, compression, elevation), painkillers (particularly NSAIDs) and, later in recovery, light stretching exercises. Particularly, immediate application of ice while elevating the area may reduce or completely prevent swelling by restricting blood flow to the area and preventing internal bleeding. Rest and preventing re-injury is essential for rapid recovery. Applying a medicated cream containing mucopolysaccharide polysulfuric acid (e.g., Hirudoid) may also speed the healing process. Other topical creams containing skin-fortifying ingredients, including but not limited to retinol or alpha hydroxy acids, such as DerMend, can improve the appearance of bruising faster than if left to heal on its own.

Very gently massaging the area and applying heat may encourage blood flow and relieve pain according to the gate control theory of pain, although causing additional pain may indicate the massage is exacerbating the injury.[10] As for most injuries, these techniques should not be applied until at least three days following the initial damage to ensure all internal bleeding has stopped, because although increasing blood flow will allow more healing factors into the area and encourage drainage, if the injury is still bleeding this will allow more blood to seep out of the wound and cause the bruise to become worse.

In most cases hematomas spontaneously revert, but in cases of large hematomas or those localized in certain organs (e.g., the brain), the physician may optionally perform a puncture of the hematoma to allow the blood to exit.[11]

Folk medicine, including ancient medicine of Egyptians, Greeks, Celts, Turks, Slavs, Mayans, Aztecs and Chinese, has used bruising as a treatment for some health problems. The methods vary widely and include cupping, scraping, and slapping. Fire cupping uses suction which causes bruising in patients. Scraping (Gua Sha) uses a small hand device with a rounded edge to gently scrape the scalp or the skin. Another ancient device that creates mild bruising is a strigil, used by Greeks and Romans in the bath. Archaeologically there is no precedent for scraping tools before Greek archaeological evidence, not Chinese or Egyptian.[12]